This invention relates in general to a pneumatic circuit for controlling an air distribution system. More specifically, the invention relates to an improvement in the adjustment mechanism shown in pending application Ser. No. 201,226, filed Oct. 27, 1980 by Douglas F. Edwards, Ronald S. Zimmer and Raymond H. Dean, now U.S. Pat. No. 4,312,475.
The heating and cooling of relatively large buildings such as office buildings is normally accomplished by passing conditioned air through ventilating ducts which direct the conditioned air to separate rooms of the building. Individual temperature control for the separate offices or other sections of the building is achieved by controlling the volume of air flow through the duct or through the air outlet which discharges the conditioned air from the duct into the room. Typically, a flow control device is provided in the duct or outlet to regulate the flow of conditioned air to an air diffuser or similar outlet device, thereby controlling the room temperature. This type of air distribution system is generally high in efficiency and low in cost since it utilizes a single large heating or cooling unit to supply several rooms or floors of the building. At the same time, there is no sacrifice in the individual temperature control for each office.
As disclosed subsequently herein, it is possible in this type of air distribution system to provide a pneumatic control circuit that controls the flow rate of conditioned air such that it is virtually independent of the main supply pressure. As a result, the fluctuations that inevitably occur in the supply pressure have no appreciable effect on the flow of conditioned air into the room that is to be heated or cooled. Although such control circuits function well for the most part, the fact that the maximum air flow is constant is sometimes detrimental to the performance of the system. In many instances, it is desirable to provide a field adjustable upper limit on the maximum air flow. It is also desirable to provide an independent factory calibration of the maximum flow rate in order to compensate for any small physical variations that are present in the mechanical components of the system.
The aforementioned Edwards et al application discloses a mechanism that effectively adjusts the maximum flow rate in an air distribution system while eliminating many of the problems inherent in needle valves and other adjustable restrictions. Nevertheless, this device has not been wholly without problems. Perhaps most notably, the angle of the cam surface is fixed for a given cam, and the device is thus sometimes incapable of effectively accommodating diaphragms that vary significantly in elasticity. The fixed cam angle can be too shallow to provide a meaningful adjustment for diaphragms that are particularly stiff, while the same cam angle can be too steep for highly flexible diaphragms.
Another problem arises when there is substantial clearance between the tube and the opening through which it extends in the top of the control. If the tube fits loosely in the opening, relatively small external forces applied to the tube can rock it about the fulcrum of the set screw and thus upset the setting and calibration of the device. For example, when the set screw is on a low area of the cam and an upward or sideward force is applied to the tube the loose fit of the tube permits it to rock or tilt upwardly about the screw tip to raise the control orifice slightly away from the underlying diaphragm. Conversely, application of a downward force tilts the lower end of the tube downwardly to move the control orifice closer to the diaphragm. In either event, the control orifice is inadvertently shifted in a manner to throw off the calibration and setting of the mechanism, and its accuracy and effectiveness suffer accordingly.
The coiled compression spring included in the device is not able to effectively resist lateral deflection of the tube, and the spring provides little assistance in countering the tendency for the tube to rock about the set screw. Actually, the spring force tends to twist the tube and thereby adds to the frictional forces when the tube slides up and down. The vertical sliding friction increases the calibration and setting hysteresis and reduces the accuracy and sensitivity of the control.
Another problem with the device disclosed in the Edwards et al application is that the "feel" differs depending upon whether the set screw is moving up or down the incline of the cam surface. When the cam is turned in a direction to raise the tube, the set screw must ride up the cam surface and considerable resistance is encountered. Lowering of the tube is accomplished without similar resistance because the set screw then moves down the cam surface and there is no spring force to overcome. The overall result is that there is unequal resistance to turning of the cam, and there is a corresponding lack of good "feel" to the cam mechanism. Unequal resistance to turning of the cam can also cause it to tilt or rock.